1. Field of the Invention
This invention relates to a liquid crystal element suited for a liquid crystal phase grating formed with a liquid crystal, or to a liquid crystal element or a device such as a focusing screen which uses a liquid crystal and is highly suitable for a photographic camera, a video camera, etc., and more particularly to a focusing screen arranged to give a desired diffusion characteristic by controlling an electric field applied to the liquid crystal in such a way as to change its configuration.
2. Description of the Related Art
Focusing screens of varied kinds using liquid crystals have been proposed They include a popularly known kind using scattering type liquid crystals by utilizing their so-called dynamic scattering mode resulting from their property of having local and temporal changes in refractive index when the liquid crystal molecules make random motions
FIG. 24 of the accompanying drawings shows in outline the conventional scattering type liquid crystal cell. The illustration includes glass substrates 241a and 241b; and transparent electrodes 242a and 242b each of which is made of a thin film consisting of indium oxide and tin oxide These electrodes 242a and 242b are applied by vapor deposition to the inner sides of the glass substrates 241a and 241b. A liquid crystal layer 244 is formed by a frame body 243 which is interposed between these electrodes 242a and 242b. A voltage is arranged to be applied to the liquid crystal layer 244 by means of a drive source 245 and a switch 246 via the pair of transparent electrodes 242a and 242b. The voltage application enables the liquid crystal cell to act as a focusing screen by scattering the liquid crystals
FIG. 25 shows the scattering state of the liquid crystals caused by the voltage application through changes occurred in the quantity of vertical transmission light when white light L is allowed to be vertically incident on the above stated liquid crystal cell.
As regards known methods for changing the diffusion characteristic of the focusing screen using a liquid crystal cell, Japanese Laid-Open Patent Application No. SHO 48-37379, for example, discloses a method of changing the whole focusing screen between a transparent state and a scattered state according to the presence or absence of an applied voltage. In another method which is disclosed in Japanese Laid-Open Patent Application No. SHO 50-115523, the diffusion characteristic is changed by adjusting the applied voltage. Generally, the scattering type liquid crystal cell is arranged to have a light scattering effect by utilizing the fact that a liquid crystal molecule group is brought into a turbulent state within the liquid crystal layer by a voltage applied thereto. The liquid crystal molecule group which gives the scattering effect measures about several .mu.m to several hundred .mu.m. When a focusing screen which consists of such liquid crystals is disposed at a part of the view finder of a camera and is arranged to be observed in a state of being magnified by several diameters, the turbulent state of the liquid crystal undesirably comes to show.
Further, there has been known a liquid crystal phase grating wherein a phase grating is formed with liquid crystals and the degree of phasic changes is controlled by voltages applied thereto. FIG. 26 shows in a sectional view the arrangement of the liquid crystal phase grating of this kind. As shown, a liquid crystal layer 264 is interposed between and carried jointly by a first transparent electrode 261 and second transparent electrodes 262 and 263.
Generally, the liquid crystal molecules give a refractive index "no" for light polarized in the direction of the major axis of the molecule and a refractive index "ne" for light polarized in the direction of the minor axis thereof. The liquid crystal molecule thus can be expressed as a spheroid having a major axis of "2ne" and a minor axis of "2no". FIG. 27 is a sectional view showing such a liquid crystal molecule.
Referring to FIG. 27, a reference numeral 270a denotes in a case where the applied voltage is zero. Another numeral 270b denotes it as in a state of having its major axis turned round toward an electric field by dielectric anisotropy. The turning degree is proportional to the intensity of the electric field applied. In the case of FIG. 27, the major axis is turned round to a degree of angle .theta.. When an incident light comes in the direction of an axis z under this condition, this incident light can be considered to be split into a polarized light component in the y axial direction perpendicular to an axis x and a light component in the x axial direction. In the case where the light is natural light, these components are in equal quantities. As apparent from FIG. 27, the polarized light component in the y axial direction remains at the refractive index "no" irrespectively of the rotation of the liquid crystal molecule. Therefore, the polarized light component in the x axial direction is alone changed accordingly as the liquid crystal component rotates. Then, a refractive index "n.theta." is obtained as a value obtained at a point where it intersects the axis x. Therefore, assuming that the orthogonal projections of "n.theta." on axes x' and z' which are obtained with the axes x and z turned round to the degree .theta. are "x.theta." and "z.theta." respectively, since the section is elliptic, the refractive index "n.theta." can be expressed as follows: ##EQU1## The refractive index "n.theta." thus changes from "ne" (.theta.=0) to "no" (.theta.=90.degree.). FIG. 28 is a graph showing the refractive index in relation to the light polarized in the direction of the major axis of the liquid crystal molecule. The relation illustrated was obtained from the a nematic liquid crystal products ZLI-1694 (ne=1.633 and no=1.503) of Merck & Company, Ltd..
The refractive index of the part to which a voltage is applied can be changed, therefore, by applying the voltage to the liquid crystal layer in such a way as to turn round the liquid crystal molecules. In the case of FIG. 26, the liquid crystal molecules in the areas 265 and 266 are changed from their initial orientation to obtain a phasic change by applying voltages to the first transparent electrode 261 and the second transparent electrodes 262 and 263. Further, in the case of this figure, the degree of the phasic change is in relation to the turning degree of the liquid crystal molecules in a manner, for example, as shown in FIG. 28. However, the liquid crystal cell shown in FIG. 26 has presented the following problems: Although there arises no problem so long as the second transparent electrodes 262 and 263 are sufficiently spaced, an undesirable phasic change would arise also in the parts 267, 268 and 269 that should have no phasic change, if the electrodes 262 and 263 are spaced less than 500 .mu.m or thereabout.
The cause for this problem is believed to be as follows: Generally the liquid crystals are dielectric, and have dielectric anisotropy, in which their dielectric constant differs in the direction of major axis and in the direction of the minor axis of the liquid crystal molecule. The arrangement to have such a dielectric matter interposed between two electrodes forms a capacitor. When a voltage is applied between the two electrodes in such a state, an electric field is generated not only between the two electrodes but it also leaks out from the end parts of the electrodes. The leaked electric field increases accordingly as the voltage applied between the electrodes increases and tends to turn round the liquid crystal molecules of parts other than necessary parts. As a result, a phasic change would arise to hinder the formation of a desired phase grating.
In view of this, there has been proposed a method for forming a minute phase grating by positively using the leak of the electric field. This method is effective particularly for a phase grating the period of which is less than about 500 .mu.m. FIGS. 29(a) and 29(b) show the examples of the conventional arrangement made according to this method. A salient feature of these examples resides in that the second transparent electrodes 262 and 263 are formed to have a sufficiently small width relative to the cycle or period of their configuration. Meanwhile, the first electrode 261 either may be homogeneously formed over the whole surface as shown in FIG. 29(a) or may be arranged in the same manner as the second transparent electrodes 262 and 263 in the form of transparent electrodes 261a and 261(b as shown in FIG. 29(b). In the case of FIG. 29(a), the electric field has leaked from the end faces of the second transparent electrodes 262 and 263 when a voltage was applied between the first transparent electrode 261 and the second transparent electrodes 262 and 263. FIG. 30 shows the leak electric field in terms of voltage.
In the case of FIG. 30, the second transparent electrodes 262 and 263 are arranged to measure 10 .mu.m in width and are linearly arrayed at intervals of 100 .mu.m. Voltages generated between the surfaces of the first and second electrodes 261, 262 and 263 when a rectangular wave of 1 KHz having 10 V in its peak-to-peak value is applied between the first transparent electrode 261 and the second transparent electrodes 262 and 263 are plotted by distances in the direction perpendicular to an electrode grating obtained at the second transparent electrodes.
Further, in this instance, the initial orientation is obtained by a rubbing treatment on an orientation film (not shown) which is formed on the transparent electrode and is homogeneously arranged in a direction orthogonally intersecting the second transparent electrodes 262 and 263. As for the liquid crystals, the nematic liquid crystal product ZLI-1694 of Merck & Company, Ltd. which is shown in FIG. 28 is employed. The refractive index distribution thus obtained is as shown in FIG. 31.
FIG. 32 shows in terms of voltage the leak of electric field which takes place from the end parts of the electrodes in the case of the arrangement shown in FIG. 29(b) using the same width and period of electrodes and the same liquid crystals as in FIG. 29(a). FIG. 33 shows in a graph the change of refractive index taking place in that instance. As apparent from comparison between FIGS. 31 and 33, the electric field leak is narrowed to permit formation of a finer liquid crystal phase grating by patternizing the shape of the first transparent electrode.
The leak quantity of the electric field varies with the dielectric constant as well as with the shape of the electrode. Therefore, the leak quantity can be lessened by using liquid crystals of a larger dielectric constant. Further, the use of the transparent electrodes may be replaced with the use of a metal material.
However, the conventional arrangement described above has presented the following problem: When a voltage is applied between two electrodes, the potential between the second electrodes 262 and 263 somewhat rises. Then, this limits the changing degree of the refractive index in some parts. In other words, the range of refractive index distribution is limited in accordance with the conventional arrangement.
Further, the diffusing degree distributable area within the focusing screen arranged according to the conventional arrangement depends on the pattern of the electrodes. Hence, it has been impossible to change the diffusing degree distribution area as desired according to the photo-taking conditions.
Further, in cases where the focusing screen is to be used for a single-lens reflex camera, the phase changing degree is considered to be sufficient if it is above 1.4 .lambda. for light having a wave length of 0.55 .mu.m. For example, in case that a nematic liquid crystal material known by the trade name of RO-TN-2108 (made by Roche Co., ne=1.78, no=1.50) is arranged to measure 25 .mu.m in liquid crystal layer thickness, the electrodes can be spaced more than 18 .mu.m.
The minimum temperature required for the action of liquid crystals is about -10.degree. C. The liquid crystal becomes a mere liquid at a temperature lower than that. Whereas, the serviceable temperature range of a single-lens reflex camera is from -20.degree. to +45.degree. C. or thereabout. A liquid crystal cell which is capable of operating as liquid crystals within this temperature range has a double-refractive index difference (ne-no) is such a small value as about 0.1 unlike the value (ne-no=0.28) of the liquid crystal material RO-TN-2108 mentioned above. For use as a focusing screen, the liquid crystal cell must be arranged to have an increased thickness or to increase the period of the electrode grating.
However, an excessive increase in the liquid crystal cell thickness would cause a shallow depth of field to falsely appear as if it is deep because of the liquid crystal focusing screen. Therefore, the thickness of the cell cannot be much increased for the performance of the lens. Meanwhile, the arrangement to increase the spacing between electrodes results in a lowered degree of focusing accuracy. The spacing distance between them, therefore, must be set within a range from 15 to 40 .mu.m or thereabout.
In view of the above stated limitations, there are no liquid crystals that are usable over the above stated operating temperature range in combination with simple grating electrodes. In other words, it is desirable to bring the maximum value of refractive index changes closer to the value of the double-refractive index difference (ne - no) possessed by the liquid crystal.